Bruce Sterling writes: (((No study of the Greenhouse Effect is quite complete without a high-tech fix. The planet is cluttered with blueprints: we can dump iron filings in the ocean, dam the Bering Straits to contain the melting ice, switch over tomorrow to nuclear everything, grow mutant algae in the high seas, and paint every roof and road white. Here, a special to WHOLE EARTH, is a fresh, untried scheme with grand vision!)))

Lots of people talk about global warming, but few offer a solution, and none are cheap. The Kyoto remedy is to reduce the emission of greenhouse gases worldwide by fiat. But stuffing corks into every smokestack and tailpipe will be expensive and politically problematic. In the US alone, cutting CO2 emissions to 80% of the 1990 level may cost $3.6 trillion (1990 dollars), 30 times Apollos price tag.

The idea of placing mirrors in space dates back to Tsiolkovsky and Tsander in the 1920s. In space, even insubstantial sunlight exerts pressure, so a mirror generates continuous thrust without any rocket fuel.

Space mirrors can also reduce the amount of sunlight striking the earth! To work as a parasol, a sail must hold itself between the Earth and the Sun for many years. The ideal spot is the L1 Lagrange point, 1.5 million kilometers inside Earths orbit on the Earth-Sun axis. A parasol will have to actively reconfigure itself to maintain this somewhat unnatural position, but this should be achievable by mid-21st century distributed automation. While providing free thrust to a space mirror, solar photons cannot complete their journey to Earth to heat us up. Intercept enough photons in space, and greenhouse gases become irrelevant!

How much shading is enough? Between the mid-16th and 17th centuries, the sunspot cycle shut down, reducing solar output by a mere 0.25%. Astronomers call this the Maunder minimum27; historians call it the Little Ice Age. The Thames River froze, European population growth stalled, crops failed, and sea ice cut off Iceland from Europe.

During the Maunder Minimum, Tycho Brahe, noted Danish astronomer, recorded winter temperatures 2.7°F below average. We note that the lower limit of IPCC's estimated Greenhouse temperature change is also 2.7°F, but in the opposite direction. Therefore, creating a mirrored Maunder Minimum should let us adjust global temperature just to our liking.

and above all, reversible (reflectors can easily be shifted to let sunshine through again).

Constructing these mirrors will be greatest industrial enterprise yet attempted by the human race (second only to two heroic centuries of digging up fossil fuel and burning it). Lifting these giant shields off the surface of the Earth is out of the question. Only offworld resources will do: lunar mining, processing and launch facilities, harvesting asteroids, and robotic manufacturing.

At Earth orbit, 1,400 megawatts of solar energy pass through every square kilometer. These mirrored sails and shields are also power generators. If just 10% of this absorbed energy is converted into electricity and beamed to Earth by microwave, each ten square kilometers of sail would displace a billion-dollar carbon-burner on the ground, eliminating Greenhouse pollution.

Together the space mirrors could provide the total planetary electrical demand projected for the year 2050, i.e., 300 quads/year. Replacing the worlds generating plant by 2050 will cost roughly $20 trillion anyway. Why not set funding aside now, for clean energy from space that also keeps us cool?

Space mirrors may not only give humanity a better climate on the ground, but also open the door to developing the solar system, a gift that will keep on giving. Mirrors have yet another historic function: silent signals across long distances. Focussed reflections can show Inquiring Minds Out There that we Earthlings have got it going on!

About the Authors

Ken Roy is an engineer working in Oak Ridge amidst relics of the Manhattan Project. Most of his writing concerns how space technology will affect the future. Notably, Ship Killers from Space, a cover story of the prestigious Proceedings of the US Naval Institute, developed the strategic implications of basing kinetic energy weapons in low earth orbit.

Robert Kennedy is president of the Ultimax Group Inc., a corporation distributed across 11 time zones from Moscow to L.A. He speaks enough languages to start bar fights in all of them. Robotics engineer, amateur historian, and jack of all trades, he spent 1994 working for the House Science Committee's Subcommittee on Space as ASME's Congressional Fellow. On the Sputnik anniversary in October 1997, he managed to make the Russian evening news. Robert telecommutes from Oak Ridge, Tennessee, where he resides with his wife, numerous cats, the occasional horse, and a yard full of trees and Detroit iron.

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